A Microscale Approach to Analyzing Interfacial Transport and Mechanics

Control of surfactant transport timescales is central to many engineering processes involving multiple fluid phases. An example is the 2010 Deepwater Horizon oil spill in the Gulf of Mexico, where a better knowledge of adsorption timescales at marine-relevant temperatures, salinities, and flow speeds would have enabled greater success in the application of dispersant at the wellhead. Conventional approaches to measuring transport parameters use millimeter-scale bubbles for which transport is diffusion controlled, leading to unreliable kinetic rate constants and unrealistic numerical values of diffusivities.

We have developed a microtensiometer device that uses microscale bubbles to directly and rapidly measure dynamic surface tension for small sample volumes.  Dynamic surface tension is commonly used to measure surfactant transport parameters, where diffusivities and kinetic rate constants are obtained via multi-parameter fits to a series of characteristic data curves. One problem with this approach is that there is no way to assess a priori which mechanisms are most important. The data offer few visual cues to discern the underlying transport mechanisms. Our work has shown that an extracted characteristic timescale can be used to establish the dominant transport mechanism at given conditions. We have used microtensiometer experiments, numerical simulations, and scaling arguments to determine the characteristic timescale for diffusion of surfactant to a spherical interface, as a function of concentration and radius. Our work has established guidelines for separating transport timescales in engineering processes.

The microtensiometer device has many advantages, including the ability to rapidly test small quantities of expensive materials, and therefore the ability to rapidly evaluate formulations. We are currently using the device to characterize complex interfaces with rich interfacial rheological behavior.

Funding: Gulf of Mexico Research Initiative (part of The Consortium for the Molecular Engineering of Dispersant Systems with Tulane University, V. John, PI, “The Science and Technology of Dispersants as Relevant to Deep-Sea Oil Releases”), Berkman Faculty Development Fund

Current Students: Anthony Kotula, Yang Guo, Matthew Reichert (advisor: Lynn Walker)

Former Students: Nicolas Alvarez, Hans Mayer

Collaborators: Lynn Walker

Related Publications

N.J. Alvarez, L.M. Walker, S.L. Anna, “A Criterion to Assess the Impact of Confined Volumes on Surfactant Transport to Liquid-Fluid Interfaces,” in press Soft Matter, accepted for publication June 21, 2012.

N.J. Alvarez, S.L. Anna, T. Saigal, R.D. Tilton, L.M. Walker, “Interfacial Dynamics and Rheology of Polymer-Grafted Nanoparticles at Air-Water and Xylene-Water interfaces,” Langmuir, 28 (2012) 8052 – 8063.

N.J. Alvarez, D.R. Vogus, L.M. Walker, and S.L. Anna, “Using bulk convection to approach kinetic-limited surfactant dynamics,” Journal of Colloid and Interface Science, 372 (2012) 183-191.

N.J. Alvarez, W. Lee, L.M. Walker, and S.L. Anna, “The Effect of Alkane Tail Length of CiE8 Surfactants on Transport to the Silicone Oil-Water Interface,” Journal of Colloid and Interface Science, 355 (2011) 231-236.

N.J. Alvarez, L.M. Walker, and S.L. Anna, “A Microtensiometer to Probe the Effect of Radius of Curvature on Surfactant Transport to a Spherical Interface,” Langmuir, 26 (2010) 13310-13319.

N.J. Alvarez, L.M. Walker, and S.L. Anna, “Diffusion-limited adsorption to a spherical geometry: The impact of curvature and competitive time scales,” Physical Review E, 82 (2010) 011604.

N.J. Alvarez, L.M. Walker, and S.L. Anna, “A non-gradient based algorithm for the determination of surface tension from a pendant drop: Application to low Bond number drop shapes,” Journal of Colloid and Interface Science 333 (2009) 557-562.

H.C. Mayer and S.L. Anna, “A Microfluidic Tensiometer,” Proceedings of the IMECE2004, 2004 ASME International Mechanical Engineering Congress, November 13-19, 2005, Anaheim, CA, IMECE2004-62096.

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